Index

Chapter 8: Zombie DNA

Let's finish writing the feedAndMultiply function.
The formula for calculating a new zombie's DNA is simple: It's simply that average between the feeding zombie's DNA and the target's DNA.
For example:

function testDnaSplicing() public {
  uint zombieDna = 2222222222222222;
  uint targetDna = 4444444444444444;
  uint newZombieDna = (zombieDna + targetDna) / 2;
  // ^ will be equal to 3333333333333333
}

Later we can make our formula more complicated if we want to, like adding some randomness to the new zombie's DNA. But for now we'll keep it simple — we can always come back to it later.

Put it to the test

1. First we need to make sure that _targetDna isn't longer than 16 digits. To do this, we can set _targetDna equal to _targetDna % dnaModulus to only take the last 16 digits.

2. Next our function should declare a uint named newDna, and set it equal to the average of myZombie's DNA and _targetDna (as in the example above).
Note: You can access the properties of myZombie using myZombie.name and myZombie.dna

3. Once we have the new DNA, let's call _createZombie. You can look at the zombiefactory.sol tab if you forget which parameters this function needs to call it. Note that it requires a name, so let's set our new zombie's name to "NoName" for now — we can write a function to change zombies' names later. Note: For you Solidity whizzes, you may notice a problem with our code here! Don't worry, we'll fix this in the next chapter ;)

pragma solidity ^0.4.25;

import "./zombiefactory.sol";

contract ZombieFeeding is ZombieFactory {

  function feedAndMultiply(uint _zombieId, uint _targetDna) public {
    require(msg.sender == zombieToOwner[_zombieId]);
    Zombie storage myZombie = zombies[_zombieId];
    _targetDna = _targetDna % dnaModulus;
    uint newDna = (myZombie.dna + _targetDna) / 2;
    _createZombie("NoName", newDna);

  }

}